I recently switched to a 42t single ring in front, running as the inside on a double crank with a 42t size chainguard protector in place of the outer ring. 9 spd ultegra, SRAM 11-28 drivetrain. yesterday broke my chain standing on a short power climb during a cx race. It's an old chain (9 months of road racing), but just wonder how much the chain line had to do with it. the chain line doesn't look bad to me. and when it broke, I was in a small gear (ie, big cog). but there were flat sections where I was cranking in the harder gears. anyone else experience broken chains after going to a single ring in front?
The chainline's as good / bad as it's going to get. You can measure things and be sure, but if you think about it, the chainline is exactly the same as it would be as a double while using the small ring. A bit of cross chain on the small cogs, but not that much.